1. Field of the Invention
The present invention generally relates to devices for monitoring or regulating electrical power supplies, and more particularly to a monitor for a standard 3-spade 120 volt electrical outlet, which assures the user that the outlet has been wired properly and has acceptable grounding properties.
2. Description of the Prior Art
When electrical outlets are installed in a home or business, it is extremely important that they be properly wired. When such outlets are miswired, they may become totally useless or, even worse, result in serious damage to persons or property. This is especially critical when dealing with sensitive electrical equipment such as computers, or when a proper ground connection is necessary to avoid the danger of electrical shock to the person using the equipment.
There are three wires in a typical 120-volt outlet, namely, the "hot" wire, the neutral wire, and the ground wire. The voltage between the hot wire and either the neutral wire or the ground wire is 120 volts AC. In a standard 3-spade outlet, the hot and neutral wires should be connected to the two flat, parallel contacts, while the ground should be connected to the rounded contact. In modern outlets, one of the ports for the parallel contacts is also smaller than the other; the smaller port is designated for the hot line, and the longer port is for the neutral. Oftentimes, however, these wires are connected to the wrong contacts, or the wiring from the main terminal does not provide the proper three lines to begin with, e.g., there may be two neutral wires and one ground wire leading to the outlet. Additionally, a fourth wire--or second hot wire--is sometimes mistakenly provided which is 180 degrees out of phase with the hot wire (or 120 degrees out of phase in a 3-phase supply), i.e., there is 240 volts between it and the hot wire (or 208 volts in a 3-phase supply). Various permutations which may be had with these four wires are shown in Table 1 in the Description of the Preferred Embodiments below.
Several devices can be used to check for proper wiring in electrical outlets. The most common of these is known as a volt-ohmmeter. This apparatus typically has a dial for selecting (i) current, voltage, or resistance measurements, (ii) the magnitude range of the electrical signal to be tested, and (iii) the kind of voltage to be tested for (AC or DC). AC voltage in an outlet may be checked by inserting two probes into the parallel ports; however, a proper reading does not necessarily mean that the outlet is wired properly. For example, the polarity may be wrong, i.e., the hot and neutral wires are reversed. Volt-ohmmeters also cannot check the static draining properties of the ground connection.
One device developed especially for testing electrical outlets, but rather simple in design, is the three-lamp tester shown in FIG. 1. This device has three separate lamps which, when deciphered according to the instructions, indicate the miswiring condition present. The device will not, however, detect the presence of two hot lines at the neutral and ground contacts when the hot contact is actually the neutral line (condition 22 on Table 1 below). Moreover, this device will erroneously give a proper wiring indication on conditions 2 and 3 in Table 1 when there is sufficient capacitive coupling of the non-connected lead to either the ground or neutral lines. This condition would be encountered, for example, if the ground lead was connected at the outlet but not at the electrical distribution panel, there being coupled capacitance between the ground line and the neutral line along the length of the supply conduit.
The closest prior art device is probably the monitor manufactured by Pilgrim Electric Company of Plainview, New York, under the brand name GAM-1 Smart Static Ground Monitor. The circuitry of this device is shown in FIG. 2. It will be appreciated that the Pilgrim device operates by blocking current flow to the lamp, rather than shunting current around the lamp as contemplated by the present invention. While the GAM-1 is useful for detecting the presence of the hot line, there are several conditions it will not detect, notably when the second hot wire is present (the 240/208 volt setting). Moreover, the GAM-1 will not detect the presence of a hot line on the ground spade (condition 22 on Table 1), which can precipitate a serious accident, especially if the ground connection is being used to discharge static from an individual via a wrist strap or other means. It would, therefore, be desirable and advantageous to devise an electrical outlet monitor which may discern any deviation from proper wiring in a 3-spade outlet, including reverse polarity, extra hot lines, and inadequate grounding characteristics.
Accordingly, the primary object of the present invention is to provide a monitor for standard 3-spade electrical outlets which assures the user of proper hot wire configuration.
Another object of the invention is to provide such a monitor which is also sensitive to the presence of an extra hot wire, including an out-of-phase wire used for 240/208 volt outlets.
Still another object of the invention is to provide a static ground monitor which confirms adequate grounding characteristics of the outlet.
Yet another object of the invention is to provide a monitor which detects the presence of open lines where neutral or ground wires should be.